This application relates to a method and reagents for using .sup.77 Se-Nuclear magnetic resonance spectroscopy to provide stereochemical assignments and to evaluate the enantiomeric purity of a composition. The application further relates to a synthetic method for preparing the reagents.
Obtaining compositions of high or at least well defined optical purity is of importance in a great many practical applications of synthetic organic chemistry. Many pharmaceuticals are most active in one enantiomeric form. Similarly, different enantiomers can have very distinctive organoleptic properties making the question of enantiomeric purity of significance in the flavoring and fragrancing arts. On a more academic level, evaluation of enantiomeric products can provide valuable insights into both conventional and enzymatic reaction mechanisms.
Over the past decade considerable effort has been spent in numerous laboratories developing new synthetic methodology in the quest for improved asymmetric synthesis. This goal has been attained in many instances through the exertion of stereochemical control to achieve optical purity. The challenge to conceive and execute a new strategy which provides the organic chemist with either optically pure starting materials or an enantiospecific total synthesis of an important natural product is paramount. While innovation is giving the synthetic chemist an arsenal of methods for asymmetric synthesis, a continuing problem has been the facile determination of enantiomeric or diastereomeric purity in natural or synthetically constructed molecules.
Typically, if the optical purity cannot be directly evaluated using physical methods, the organic chemist alters the molecule, frequently in a multistep manner, to a derivative whose rotation is a reported literature value. This method, although tedious and time consuming, gives reliable and consistent results if the following caveats are recognized: 1. The solvent must be the same as the one in which the rotation was originally reported (e.g., verbenol, a natural product, is levorotatory in chloroform and dextrorotatory in acetone or in methanol); 2. Trace impurities can cause errors in polarimetric determinations; and 3. Reported .sup.[.alpha.] D values are not uniformly accurate, e.g., exo-brevicomin reportedly has rotations of (in ether) +69,30, -69,70, and +72.40, -73.60. Clearly, the data obtained from optical rotations must include the exact conditions in which the determinations were made.
For some types of compounds, chromatographic methods have been suitable for the determination of the enantiomeric purity. Francotte et al., Helv. Chim. Acta. 70:1569 (1970). Separation is possible by using a chiral stationary phase or by converting the enantiomers into diastereomers, followed by chromatographic fractionation on an achiral stationary phase. In some cases this method has been excellent results but, it basically remains a trial and error procedure.
An an alternative to optical and chromatographic approaches, techniques have been developed to determine the ratios of enantiomers using nuclear magnetic resonance spectroscopy (NMR). One such method involves the use of chiral europium shift reagents (e.g., Eu(tfc).sub.3 or Eu(hfc).sub.3) for NMR detection of ratios of enantiomers. Sullivan, G. R., in "Topics in Stereochemistry," Eliel et al. eds., Wiley-Interscience, New York (1978), page 263. This method is successful due to the fact that solution diastereomers are formed by the interaction of the chiral shift reagent with some functional group of the enantiomers (e.g., a ketone functional group). The procedure is excellent because it is direct, no chemical modification is necessary, and it is relatively inexpensive. On the other hand, applicability of the procedure is limited because it requires the chiral center in question to be proximal (usually one or two atoms removed) to the chiral shift reagent, thus forming contact diastereomers. If the chiral center is distant to the shift reagent then this method fails. In addition, use of high concentrations of these shift reagents causes line broadening in the NMR spectrum which makes interpretation difficult.
In another approach, chiral solvent has been used for the NMR determination of enantiomeric ratios. Again, this is a trial and error method and the cost of these solvents can be prohibitive.
Ratios of enantiomers in some compounds can be evaluated after the preparation of diastereomeric derivatives, such as Mosher's acid (.alpha.-methoxy-.alpha.-trifluoromethylphenylacetic acid) and their subsequent evaluation by NMR spectroscopy. Dale et al., J. Amer. Chem. Soc. 95:512 (1973); Williams et al., J. Amer. Chem. Soc., 110:1547 (1988); Vandewalles et al., Tetrahedron 42: 4035 (1986). Enanatiomeric ratios of secondary alcohols and amines usually can be obtained from the .sup.19 F NMR of their corresponding Mosher's ester derivatives (structure I). ##STR2## This method relies on the fact that the ester derivatives are diastereomeric and the .sup.19 F nucleus has adequate sensitivity to detect their physical differences. Note, however, that the fluorine nucleus can be no closer than, nor further than five bonds removed from the Mosher's ester chiral center. In fact, even at five bonds sometimes resolution of the resonances is not great enough for quantitative measurements. Another drawback of this method is the cost of Mosher's acid (1 gram cost $30-$40), even though the acid is recoverable via hydrolysis.
In a related approach, Johnson has recently reported the use of .sup.31 P NMR spectroscopy in the determination of enantiomeric purities of alcohols and amines via the use of the oxazaphospholidine-2-sulfide (commercially available from Fluka). Johnson et al., J. Amer. Chem. Soc. 106:5019 (1984) However, several limitations exist with this method. For example, this method is only useful for secondary alcohols and amines. The recycle time is reported to be 60 seconds, which for "normal" concentrations requires prohibitively long NMR experiment times. The sensitivity is marginal as manifested in the reported .DELTA..delta. (0.100-0.350 ppm).
Both Mosher's acid and oxazaphospholidine-2-sulfide possess the same drawback; only secondary alcohols and amines can be evaluated with these reagents which severely limits these detection methods.
Finally, Trost et al. has reported on the use of O-methyl-mandelate esters to determine enantiomeric excesses at chiral centers by .sup.1 H NMR spectroscopy quantitation of the methoxy resonances. This method has the added advantage of assigning the absolute configuration of the chiral center, in most cases, by comparison of the chemical shifts as to whether the major methoxy .sup.1 H resonance is deshielded or shielded. An added bonus is that this method uses routine .sup.1 H NMR spectroscopy in which every student involved in total synthesis and the development of new synthetic methods or organic chemistry becomes proficient. Again, however, only secondary alcohols and amines can be evaluated, which severely limits this detection method.
It is an object of the present invention to provide an approach to evaluation of enantiomeric ratios which is comparatively easy to use and provides superior versatility in the type of compounds which can be evaluated.
It is a further object of the invention to provide an approach to evaluation of enantiomeric ratios which has high sensitivity.